1. Field of the Invention
This invention relates to a high-pressure mercury lamp, and particularly to a high-pressure mercury lamp with at least 0.15 mg/mm3 of mercury sealed into the discharge vessel so as to have a pressure of well over 100 atmospheres when lit for use as a back light in liquid crystal projectors or projection-type liquid crystal display equipment.
2. Description of Related Art
Projection-type liquid crystal display equipment is required to project an image on a rectangular screen, evenly and with good brightness, efficiency and color characteristics. For that reason, metal halide lamps, in which are sealed mercury and a metal halide, are commonly used as light sources. In recent times, these metal halide lamps have come to have very small inter-electrode gaps, making them smaller and more nearly point light sources.
Against this backdrop, high-pressure mercury lamps, having well over 100 atm. of mercury vapor pressure when lit, have been proposed to replace metal halide lamps. Making the mercury vapor pressure higher is intended to suppress the spread of the arc and make the light output even higher. Examples include U.S. Pat. No. 5,109,181 (JPO kokai patents H2-148561) and U.S. Pat. No. 5,497,049 (H6-52830).
For high-pressure mercury vapor lamps of this sort to achieve high brilliance, high efficiency and good color characteristics, it is necessary to have a mercury vapor pressure well over 100 atmospheres, and so at least 0.15 mg/mm3 of mercury must be sealed into the discharge vessel. In this way, after the lamp has been started up, mercury that is reserved in the light emission tube in liquid form is heated and vaporizes; several minutes of time is required for the vapor pressure to rise and bring the light output to the desired level. This phenomenon is one which did not arise to a noticeable extent in low-pressure mercury lamps or in conventional high-pressure mercury lamps in which the mercury vapor was not high enough (internal pressure up to 80 atmospheres when lit, for example.) Moreover, a glow discharge generally occurs when a discharge lamp is started up, but the high cathode drop voltage causes sputtering of the tungsten that makes up the cathode; the tungsten adheres to the inside wall of the discharge vessel, and the light output of the lamp is reduced. That is a problem in terms of the longevity of short-arc, high-pressure mercury lamps
Accordingly, the problem to be resolved by this invention is the provision of a high-pressure mercury lamp luminescent device, with a method of ignition, that shortens the rise time of the light output and enables prevention of a glow discharge when the lamp is started up.
This problem is solved in accordance with a first aspect of the present invention by using a high-pressure mercury lamp luminescent device with a pair of electrodes placed opposite one another within a discharge vessel made of quartz glass with seals formed on both ends and at least 0.15 mg/mm3 of mercury sealed into the discharge vessel, in which there is a means of increasing the temperature of the outer wall of the luminescent portion of the discharge vessel to at least 100xc2x0 C. before the high-pressure mercury lamp is lit.
In accordance with one specific form of the invention, the high-pressure mercury lamp luminescent device with a pair of electrodes placed opposite one another within a discharge vessel made of quartz glass with seals formed on both ends and at least 0.15 mg/mm3 of mercury sealed into the discharge vessel, has a conductive heater wrapped around the seals of the discharge vessel and there is a means of passing electricity through the conductive heater and thus controlling the temperature of the outer wall of the luminescent portion of the discharge vessel at or above the desired temperature before the high-pressure mercury lamp is lit. More specifically, the conductive heater is wrapped around one seal of the discharge vessel, then straddles the luminescent portion of the discharge vessel by means of a metallic wire and wraps around the other seal, and the conductive heater being connected electrically to one of the external leads of the high-pressure mercury lamp, especially, the external lead on the cathode side of the high-pressure mercury lamp.
Furthermore, in accordance with another feature of the invention, a lamp power supply circuit provided to ignite the high-pressure mercury lamp, is used to supply power to the heater to increase the temperature of the outer wall of the luminescent portion of the discharge vessel before the high-pressure mercury lamp is lit.
In accordance with another aspect of the invention, a heater is wrapped around at least the seal on the cathode side and one end of the heater faces the cathode through the seal, the other end of the heater being connected electrically to the cathode, such that when power is supplied to the heater, the high potential side of the heater voltage is impressed on the other end of the heater.
The invention also provides the possibility of applying heating before the high-pressure mercury lamp is lit and while it burns.
In accordance with yet another feature of the invention, by the lamp power supply circuit and the heater power supply circuit being powered by the same power source, the output from the heater power supply circuit can be controlled such that the total power output of the two circuits does not exceed a specified value.
Furthermore, a detection means can be provided that detects the temperature of the luminescent portion, and stops the supply of power to the heater when the temperature passes a specified value.
With a method of lighting a high-pressure mercury lamp luminescent device with a pair of electrodes placed opposite one another within a discharge vessel made of quartz glass with seals formed on both ends and at least 0.15 mg/mm3 of mercury sealed into the discharge vessel according to the invention, it is possible to increase the temperature of the outer wall of the luminescent portion of the discharge vessel to at least 100xc2x0 C. before the high-pressure mercury lamp is lit.
With this invention, it is possible to shorten the rise time of the light output of the high-pressure mercury lamp, and also to prevent the glow discharge that occurs when a high-pressure mercury lamp is lit. Specifically, a high-pressure mercury lamp within which at least 0.15 mg/mm3 of mercury is sealed, and the outer surface of the luminescent portion of the discharge vessel is heated to at least 100xc2x0 C. before lighting, so that it is possible to fully vaporize the mercury in the discharge vessel before ignition. Following dielectric breakdown when the lamp is lit, the mercury vapor pressure within the discharge vessel will have risen adequately. It is therefore possible to shorten the rise time of the light output of the high-pressure mercury lamp, and to prevent the glow discharge that occurs when a high-pressure mercury lamp is lit.
As a heating means, a conductive heater is wrapped around the seals of the discharge vessel and electricity is passed through it to constitute a conductive heater. In this way, it is possible, with a simple structure, to control the temperature of the outer wall of the luminescent portion of the discharge vessel at or above the desired temperature, which is at least 100xc2x0 C., before the high-pressure mercury lamp is lighted.
The heating means can be constituted by having a conductive heater wrap around one seal of the discharge vessel, then straddle the luminescent portion of the discharge vessel by means of a metallic wire and wrap around the other seal, with the conductive heater connected electrically to one of the external leads of the high-pressure mercury lamp. In this way it is possible to make the metallic wire that straddles the luminescent portion work as a trigger wire before lighting. In other words, the passage of electricity to the conductive heater is stopped prior to lighting, but the heater is connected electrically to one of the external leads, the dielectric breakdown voltage that lights the discharge lamp is impressed on the metallic wire as well, and that can easily work as a trigger wire and begin the lighting. That is, by heating the inside of the luminescent portion in advance, before starting up the lamp, it is possible to vaporize the mercury inside, and thus, have the metallic wire, which connects to the conductive heater used for heating, function as a trigger wire.
By having the conductive heater connected electrically to the external lead on the cathode side of the high-pressure mercury lamp, to impress on the outer surface of the seal at the base of the cathode the same potential that is on the external lead when the lamp is burning steadily, it is possible to enhance the lamp""s resistance to pressure.
With a heater installed to increase the temperature of the outer wall of the luminescent portion of the discharge vessel to at least 100xc2x0 C. before the high-pressure mercury lamp is lit, and with the power to the heater is supplied by the lamp power supply circuit, it is possible to vaporize the mercury within the discharge vessel beforehand, and to have the light output of the high-pressure mercury lamp rise quickly when the lamp is lighted. It is possible, moreover, to effectively prevent the unwanted glow discharge that is liable to occur when the lamp is ignited. In addition, there is no need for separate circuitry to heat the heater, and so a reduction of cost can be realized.
When the heater is wrapped around at least the seal on the cathode side of the electrodes, and one end of the wrapped heater faces the cathode through the seal, the other end of the heater being connected electrically to the cathode, when power is supplied to the heater, the high potential side of the heater voltage is impressed on the other end of the heater. Therefore, the anode has a higher potential than the facing end of the heater, which makes it possible to prevent deterioration of the lamp""s resistance to pressure.
By having the heater power supply circuit installed to heat the outer wall of the luminescent portion of the discharge vessel to at least 100xc2x0 C. before the high-pressure mercury lamp is lit and while it bums, in this way, it is possible to have the lamp power supply circuit and the heater power supply circuit operate independently and simultaneously. Thus, while the lamp lighting operation is in progress, power can be passed through the heater and continue to raise the temperature within the discharge vessel. This hastens the vaporization of the mercury, and can further hasten the increasing brightness of the high-pressure mercury lamp.
When the lamp power supply circuit and the heater power supply circuit are powered by the same power source, and the output from the heater power supply circuit is controlled such that the total power output by the two circuits does not exceed a specified value, it is possible to hold down the power supply capacity of the power source, to reduce the size and weight of the equipment, and to lower the cost. If the power supply capacity of the power source is held down, the time required to reach the final standard brightness is lengthened, but there is hardly any change in the time needed for the brightness of the high-pressure mercury lamp to reach a practical light output.
With a detector is installed to detect the temperature of the luminescent portion, and stop the supply of power to the heater when the temperature reaches a specified value, it is possible to control the feed of power to the heater, so that a given amount is supplied in the event that operation begins when the high-pressure mercury lamp has begun to cool off, and it is possible to shorten or completely eliminate the feed of power to the lamp in the case of re-ignition a relatively short time after it has been extinguished. Thus, the wasted consumption of power is suppressed, and it is possible to prevent the deterioration of heater longevity that would result from melt-through and oxidation as would result from excessive heating of the heater.